Looked at a job today that we don't have much experience on, that is replacing a casement style metal window with a new vinyl window. The metal fin protudes into the window space about 1" all the way around. It would be a mess to cut the metal out to make it flush with the opening.
I was thinking to take a 2x6 (a full 2" board would be better but not sure how available) and use a router on the back of the board to accomodate the fin. Mount this 2x6 all the way around the opening, install window, and then wrap the outside and trim the inside of the 2x6.
Easier way? This seems pretty labor intensive.

Thanks,
dave